1. Field of the Invention
The invention relates generally to fuel cell power generation systems operating on natural gas as a fuel, and a method to reduce sulfur content of the natural gas as it enters the fuel cell.
2. Background Information
Fuel cell systems incorporating means to reduce sulfur content of feed fuel are well known and taught for example in U.S. Pat. No. 5,413,879 (Domeracki, et al.). There, natural gas or coal derived fuel gas was pumped through a fuel pre-heater and then directed, at 400° C., to a desulfurizer containing a bed of sulfur sorbent, to reduce sulfur context to less than 0.1 ppm (parts/million), after which it was then passed to a solid oxide fuel cell generator (“SOFC”).
The use of untreated natural fuel gas, diesel fuel, sulfur containing coal derived fuel gas, or other sulfur containing fuel gas, all herein defined as “sulfur containing fuel gas”, for fuel cell feed, has provided many problems including carbon deposition on many fuel cell generator components and sulfur degradation of the fuel electrode. While natural gas can contain low sulfur content, in most cases highly odorous sulfur compounds such as mercaptans are added to permit detection of gas leaks.
Reforming the fuel gas, defined as breaking down larger carbon molecules to smaller components such as C1–C2 hydrocarbons, predominantly methane, H2 and CO has been performed, usually with a steam component, internally and/or externally by endothermic reaction in catalyst beds of magnesium oxide, nickel, or platinum supported on sintered aluminum, as taught in U.S. Pat. No. 5,527,631 (Singh, et al.). Sulfur treatment of sulfur containing fuel gas was specifically dealt with by Singh, et al., in U.S. Pat. No. 5,686,196. There, diesel fuel, having a sulfur content of about 0.5 wt. %, was mixed with hydrogen at low pressure, pressurized, evaporated, and then the mixture passed to a hydrodesulfurizer consisting of a Co—Mo catalyst and a ZnO reactive metal bed, where organic S was converted to H2S and then reacted to form ZnS, to reduce sulfur content to levels of about 1.0 ppm–0.2 ppm. The desulfurized fuel was then passed to a reformer, a condenser, and then to a hydrogen separator made of metallic or polymeric material, preferably Pd—Ag on a nickel support, or a semi-permeable polysulphone hollow fiber membrane. These materials pass the hydrogen through and then back to the diesel fuel/hydrogen mixer, and pass the rejected, reformed fuel to an SOFC. This process was quite complicated and required costly regeneration or disposal of the catlyzed-reactive hydrodesulfurization bed.
Other attempts to solve sulfur poisoning of SOFC fuel electrodes were made by Isenberg in U.S. Pat. Nos. 4,702,971 and 4,812,329. There, the fuel electrode itself, a nickel particle-cermet structure on the exterior of the SOFC was coated with separate porous, gas permeable oxygen-ionic-electronic conductor material, such as doped ceria or doped urania which reduces absorption of sulfur species on the electrode active sites. This process required depositing a delicate 0.5 micrometer to 20 micrometer film and added an extra process step and an additional expense to SOFC costs. Ruka, et al., in U.S. Pat. No. 5,021,304 improved the Isenberg coating process, using a separate layer of very small nickel particles as a surface for the doped cerea or doped urania, which made the process even more complicated. What is needed is a process to eliminate the need to apply additional fuel cell layers to the external electrode. Also, replenishing sulfur sorbent, from 6 to 12 times a year, can be very expensive in a commercial process, so there is a need for improved sorbents or other means to reduce these costs.